feat(voice): per-user TTS voice selection via /목소리

Until now every TTS message was synthesized with VoiceType.가람
regardless of who spoke. This adds a user-scoped voice preference
persisted in SQLite so each member can pick their own voice and
keep it across bot restarts.

Changes
- db/schema.sql: add nullable `voice_type` column to `users`.
- src/utils/Database.ts:
  - run a PRAGMA-driven ALTER TABLE migration so existing DBs gain
    the column without dropping data.
  - add `DB.user.setVoice(guildId, userId, name, voiceType)` that
    upserts the row.
- src/classes/TTSClient.ts:
  - export `DEFAULT_VOICE` (= 가람).
  - resolve the speaking member's stored voice in `tts()` and
    thread it through `getSource()` instead of hardcoding 가람.
  - validate stored slug against `VoiceType` so stale/unknown
    values silently fall back to the default.
- src/commands/voice.ts (new):
  - `/목소리` slash command shows the user's current voice and a
    StringSelectMenu of all `VoiceType` entries (현재 + 이전 보이스
    모두). Selection writes to `users.voice_type` and confirms
    ephemerally.
  - defensively creates a guild row if `/tts channel register`
    hasn't run yet, to satisfy the ON DELETE CASCADE FK.

Deploy
Run `npm run prod` after pulling so Discord sees the new
`/목소리` command. No env or config changes required.

db/db.d.ts

@@ -8,4 +8,5 @@ export interface UserType {
   guild_id: string;
   id: string;
   name: string;
+  voice_type?: string | null;
 }